1. Field of the Invention
The present invention relates to a dimmer triggering circuit for low-load applications, e.g. LED-based light sources. The invention further relates to a dimmer system comprising such a dimmer triggering circuit.
2. Description of the Related Art
Generally, phase-controlled dimmers comprise a triode for alternating current, further referred to as a triac. A triac is a bidirectional switch which can conduct current in either direction when it is triggered, i.e. turned on. It can be triggered by either a positive or a negative voltage being applied to its gate electrode, i.e. when a small current is applied to its gate. This current only needs to be applied for a short period of time, i.e. in the order of microseconds. In other words, the triac needs to be triggered or ‘fired’. Once triggered, the device continues to conduct until the current through it drops below a certain threshold value, such as at the end of a half-cycle of alternating current (AC) mains power supply, also referred to as a zero-crossing. As a result, the triac then ‘turns off’.
These dimmers work well for dimming incandescent light bulbs which draw comparatively high currents. When these dimmers are used with smaller loads, such as light sources based on light-emitting diodes (LEDs), various problems are encountered. This is a particular problem when replacing standard incandescent light bulbs with LED retrofit light bulbs in situations where a conventional triac dimmer has been installed for use with the light bulb.
An LED light source may not draw enough current to permit the triac in the dimmer to turn on as required, resulting in an inability to dim the light or erratic operation of the dimmer. A small resistive load on the dimmer may cause an oscillation of the voltage at the dimmer output caused by multiple firings of the triac, resulting in incorrect dimming operation. At low dimmer settings the LED driver circuit may toggle on and off, causing a brief flash of light from the LED light source. Furthermore, the human eye perceives light intensity generally according to a logarithmic curve, whereas an LED has an almost linear response, the emitted light intensity being approximately proportional to the current flowing through the LED. When operated with a conventional dimmer, an LED light source will not appear to dim smoothly and the variation in perceived light intensity has no intuitive relation to the dimmer's knob position. Small variations in the supply voltage may also result in visible flickering of the light emitted by the LED light source.